Protective device.



G. H. STEVENSON.

PROTECTIVE DEVICE.

APPLICATION FILED NOV 20. 1916.

Patented-July 9, 1918. 2 SHEETS-SHEET i Lmm G. H. STEVENSON.

PROTECTIVE DEVICE.

APPLICATION FILED NOV: 20. 1916.

1 ,%7 1,79%, Patented. July 9, 1918;

2 SHEETS-SHEET Z.

LENGTH 0.05 mm.

GAP LENGTH 0.5mm

Volts 2200 50 AIR PRESSURE M.

//1 van fax Gem ye /7. Sfe ve/7'5 0/7. fly

" 'A/fy.

' 4o tween thesame two or GEORGE H. STEVENSON, OF'MOUNT VERNON, NEWYORK, ASSIGNOR T WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF

YORK.

NEW YORK, N. Y., A.

CORPORATION OF NEW PROTECTIVE DEVICE.

larly to protective devices of the vacuum The present protective devicesofthe vacuum type used for telephone circuits comprise essentially twoelectrodes separated by a short gap. 'It has been found that, withincertain limits of the length of the gap, the

maintenance cost of these devices decreases as the gap is enlarged, forthe reason that 7 there will be less tendency to short-circuit bypitting and burning of the electrodes. However, a gap which is largeenough to give low maintenance cost has had the disadvantage of notproviding for the protection of the circuit if the gas pressure isincreased beyond acertain value.

The general object of this invention is to provide a protective devicewhichwill have low maintenance cost and which, in the event of thepressure being increased,- will still protect the circuit. 1

This result is accomplished, according to the present invention, byproviding a comparatively large principal or primary gap etween twoelectrodes which, as explained above, will give low maintenance cost,and

a smaller secondary or auxiliarygap bedifierent electrodes which willprotect the circuit if the pres- 'sure is increased.

i The invention will be readily understood from the followingdescription taken in connection with the accompanying drawings -whichshow some of the which the invention may -'-in which- Figure 1 is anelevation partly in section be embodied and showln% the invention in itselementary 1 form; gs. 2, 3, 4; and 5 .areelevations partly 'n sectionof difi'erent embodiments.

. of the vention characterized respectively. by diverging electrodes,concentric electrodes, spherical faced electrodes and flat various formsin Specification of Letters Patent. 7

Patented July 9, 1918.

Application filed November 20, 1916. Serial No. 132,448.

electrodes; and Fig. 6 is a graphical representation of the relationexisting between pressure, sparking potential and the sparkgap of aprotective device.

Paschens law, upon the application of which the operation of the presentprotective device depends, states that the pressure corresponding to theminimum potential which will cause an arc to be struck betweenelectrodes surrounded b a certain gas, usually called the criticapressure, is very nearly inversely proportional to the distance betweenthe electro es.

The relation existing between these various factors will be readilyunderstood by reference to the curves shown in Fig. 6. The data fromwhich these curves are plotted is based upon the values taken fromTownsends- Electficity in Gases, page 361 (published by the OxfordUniversity Press, 1915), which represents a summary of the experimentalwork by W. R. Carr on the sparking potential of gases at low pressuresand in uniform electric fields. The values given for the sparkingpotential in air between parallel planes at various pressures, have beenrecomputed according to Paschens law for two gaps of lengths of 0.5 mm.and

0,05 mm.,-respect1vely. The curves shown are for air-gaps of theselengths. It will be seen that at a pressure of about 50 mm. of

mercury, the sparking otential of the two gaps is approx1mately-t esame. "As the air pressure. is decreased from this value, the sparkingpotential of the shorter gap increases quite rapidly. In the case of thelonger gap, however, as the pressure is e- 'creased the sparkingpotential decreases gradually until the critical pressure of about 10mm. of mercury is reached, wherefurther decrease in pressure causes thesparking potential of that gap to increase very' rapidly.

Now if a device is provided in which the electrodes are so arranged thatthere will be both a long and a short spark-gap, (for example, of thelengths for which the curves shown in Fig. 6 are plotted), it is evidentthat the pressure in the container can be adleakage or other causes,

'- creased to approximately 50 mm. of mercury, whereupon the dischargewill also take place across the shorter gap and with further increase ofpressure will prefer that gap and will take place at a graduallydecreasing sparking potential. Wlth a secondary or auxiliary gap of theshort length above mentioned, 0.05 mm., the arc will continue for only avery short time before the pittin of the electrodes will cause bridging0 the gap, thereby protecting the apparatus on the line from any furtherdisastrous efiects due to abnormal voltages, and indicating that thearrester should be renewed.

The embodiments of the invention hereinafter described are so arrangedas to operate upon the principle just explained. These devices areparticularly adapted for the protection of telephone, telegraph andsignal circuits, andin practice the length of the primary or principalgaps should be from 0.5 mm. to 1.5 mm., and the length of the secondaryor auxiliary gaps should be from 0.05 mm. to 0.1 mm., while the pressureto which the container should be exhausted should be approximately thecritical pres sure of the primary or principal (that is the longer) gap,say from 5 mm. to 20 mm. of mercury.

In the elementary form, the device as shown in Fig. 1 consists of threeflat metallic electrodes 1, 2 and 3 arranged parallel to one anotherwithin a container 4, which has been evacuated to the desired pressurein any well-known. manner. Leading-in wire 5 is connected to electrodes1 and 3, and leading-in wire 6 is connected. to electrode 2. The threeelectrodes are spaced apart from each other so as to form the larger gapbetween electrodes 2 and 3 and the smaller gap between electrodes 1 and2.

Figs. 2, 3 4 and 5 show how the different length spar gaps may besecured in various forms of vacuum protective devices.

Fig. 2 shows the invention embodied in a structure characterized bydiverging electrodes. The evacuated container 7, within which theelectrodes are mounted, may be of glass formed and evacuated in theusual manner and cemented to a suitable base 8. The electrodes 9 and 10are two flat pieces of metal bent in the manner shown, so as to slightlydiverge at their up er ends. The lower ends are also bent so t at theleading-in wires 11 and 12 can easily be soldered to them at 13 and 14respectively. The electrodes are held in proper relation to each otherby two metallic disks 15 and 16, and an insulating disk 17, which issomewhat larger than the metallic disks and has an opening at itscenter. Two insulating disks 18 and 19 are placed respectively on theoutside of electrodes 9 and 10. Over all and binding the whole assemblytogether are two metallic bars 20 and 21, crossing the electrodes andengaged ateach end by a bolt. One of these bolts is shown at 22. Theprimary or principal gaps are between the upper ends of the electrodes 9and 10, and the secondary or auxiliary gap is shown at 23. In evacuatingthe container, it is diflicult to adjust the pressure to the exactcritical value for a certain length gap. These diverging electrodes 9and 10, in eiiect, furnish a plurality of gaps which make it easy tosecure a pressure which is at or near the critical pressure for one ofthe gaps.

Fig. 3 represents a vacuum protective device with a container 24 whichis cemented to a suitable base 25. Within the container 24 is a hollowglass stem 26 upon which concentric cylindrical electrodes 27 and 28 aresupported. Leading-in wires 29 and 30 are connected to the electrodes at31 and 32.

The electrodes are separated from each other and held in operableposition by insulating disks 33 and 34. Insulating disk 33 has openingsin it which provide for the secondary gap 35. The primary gap is betweenthe electrodes 27 and 28 at 36.

Fig. 4 represents a vacuum protective device, comprising a container 37in which are electrodes 38 and 39 having opposing spherical faces. Theelectrodes are held in a proper position by means of a framework,substantially as shown in the drawing, which is supported'within thetube by two metal members 40 and 41 projecting, respectively, through anend of the tube, and being soldered to terminal caps 42 and 43 which aresuitably cemented to the container. The electrodes thus provide aplurality of parallel spark-gaps of gradually varying lengths and makeit easy to secure a pressure which i approximately the critica pressurefor one of the longer gaps.

Fig. 5 represents an embodiment of the invention comprising a container44 in which are supported, in a manner similar to that previouslyexplained in connection with Fig. 4, two flat metal electrodes 45 and46. Electrode 46 is bent at 47 and 48 so as to form a raised or bulgingsection near its center. Between the electrodes at their center isplaced a disk of insulating material 49 which has a hole in its center,thus forming the secondary or auxiliary gap 50. The ends of theelectrodes are spaced apart, thus providing the primary or larger gapsat 51 and 52. On the outside of each electrode are placed pieces ofinsulating material 53 and 54. Over all and binding the whole assemblytogether are two metallic bars 55 and 56 crossing the electrodes andengaged at each end by a bolt. One of these bolts is shown at 57.

The invention may be embodied in many other forms of protective devicesthan those described and is, therefore, not limited to the forms hereindisclosed.

What is claimed is:

1. In a protective device, a container, and electrodes therein providinga plurality of spark gaps of difi'erent lengths, said electrodes beingsurrounded by gas at substantially the critical pressure of one of thelonger gaps. 1

2. In a protective device, a container, and electrodes therein providinga plurality o1 spark gaps of lengths varying from 0.05 mm. to 1.5 mm.,said electrodes being surrounded by gas at substantially the criticalpressure of one of the longer gaps.

3. In a protective device, a container, and electrodes therein providinga plurality of spark gaps of lengths varying from 0.05 mm. to 1.5 mm.,said electrodes being surrounded by gas at a pressure from 5 mm. to 20mm. of mercury.

4. In a protective device, a container, and electrodes therein providingprimary or principal gaps having a length from 0.5 mm. to 1.5 mm. andsecondary or auxiliary gaps having a length of from 0.05 mm. to 0.1 mm.,said electrodes being surrounded by gas at a pressure of from 5 mm. to20 mm. of mercury.

5. I11 a protective device, a container, and electrodes thereinproviding longer primary or principal gaps and shorter secondary orauxiliary gaps, said primary gaps being of length such that With acertain gas pressure in said container abnormal voltage discharges Willtake place across said primary gaps, and said secondary gaps being oflengths such that when the gas pressure is increased beyond a certainvalue abnormal voltage discharges will take place across said secondarygaps.

6. In a protective device comprising a partially evacuated container, inWhich is arranged a spark gap having a minimum sparking potential atsubstantially the pressure in said container, means providing anauxiliary spark gap in parallel With said principal gap and having itsminimum sparking potential at a higher gas pressure than said principalgap, said minimum sparking potential of the auxiliary gap being lessthan the sparking potential of said principal gap at the same pressure.

In Witness whereof I hereunto subscribe my name this 18th day ofNovember, A. 1)., 1916.

GEORGE E. STEVENSON.

